foo.c: In function 'MergeSubs':
foo.c:61:6: warning: unused variable 'mark' [-Wunused-variable]
  int mark;
      ^~~~
foo.c: In function 'main':
foo.c:13:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d %d", &n1, &n2);
  ^~~~~~~~~~~~~~~~~~~~~~~~
foo.c:18:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d", &a[i]);
   ^~~~~~~~~~~~~~~~~~
foo.c:22:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d", &b[j]);
   ^~~~~~~~~~~~~~~~~~

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